A video signal typically includes vertical display intervals that comprise a plurality of horizontal line intervals, e.g. 525 lines per vertical interval in NTSC video systems. A portion of each vertical interval is usually designated as a vertical blanking interval. The vertical blanking interval may span a plurality of horizontal line intervals, e.g. in excess of 20 horizontal line intervals. The beginning of each vertical and horizontal interval is identified by respective vertical and horizontal sync pulses that are included in a composite video signal.
The content of the video signal during blanking intervals is usually not intended for display as part of the normal video image. The lack of image information in blanking intervals makes it possible to insert auxiliary information, e.g. teletext and closed caption data, into blanking intervals. The standards for each type of auxiliary information specify the positioning of the information within a vertical blanking interval. For example, the present closed captioning standard (see e.g., 47 CFR .sctn..sctn.15.119 and 73.682) specifies that digital data corresponding to ASCII characters for closed captioning must be in line 21 of vertical blanking.
An approach to recovery of auxiliary information is to accurately identify a specific line interval, e.g. line 21, containing auxiliary information during a vertical blanking interval by counting horizontal sync pulses. For example, a horizontal line counter could be initialized by a vertical sync pulse and clocked by horizontal sync pulses. Ideally, the count value would then represent the line number. However, in a non-ideal environment, e.g. a television receiver chassis, the described simplistic line-counter approach may be unreliable. As an example, noise introduced by deflection circuits related to image display apparatus, e.g. a cathode ray tube (CRT), and temperature effects may produce varying timing relationships, e.g. time delays or jitter, between versions of horizontal and vertical sync that might be used to control a horizontal line counter. Jitter in combination with any delay between horizontal and vertical sync may introduce a critical race condition between initialization of a horizontal line counter by vertical sync and clocking of the line counter by horizontal sync. A critical race condition may cause the line count to be incorrect and unpredictable. For example, in one field the count value may correctly identify line 21 while in another field the count value may be 22 when line 21 is actually being received. Thus, the described approach may not reliably identify a particular horizontal line as desired for the purpose of extracting auxiliary data from a video signal.
The described potential for line count error is particularly significant in regard to closed caption decoders that are included internal to a television receiver. Set-top decoders are external to the receiver chassis and may not exhibit the above-described noise and temperature problems. Graphics systems, e.g. on-screen display (OSD), may involve line number identification. However, minor line count errors, such as the above-described error of 1, may produce only minor shifts of an OSD image, not data loss. Line count errors of 1 will result in a loss of closed caption data because the data appears during line 21 only. Frequent line count errors may produce an unacceptable degree of closed caption data loss.